In a Frequency Division Duplex (FDD) application field, a general wireless communication system intensively outputs a signal of a transmitter of a wireless terminal in a weak electric field of a receiver in the wireless terminal (hereinafter, a wireless receiver or a receiver), for controlling power. Therefore, the receiver is vulnerable to interference by the transmitter in the weak electric field and thus, may use an external Radio Frequency Filter (RF filter) such as a Surface Acoustic Wave Filter (SAW Filter), as a separate component element, to remove the interference.
There may be various factors that cause the interference. In the specifications, a scheme of removing a mismatch which is one of the factors that cause interference in the wireless communication system will be described. Conventionally, as a scheme of removing a mismatch, a scheme of including an auxiliary circuit in a signal amplifying path identical to a signal amplifying path of a main amplifier so as to remove a mismatch, a scheme in which an output end of a receiver feeds a current voltage back to an input/output intermediate tap so as to partially correct a mismatch, and the like have been proposed.
However, the conventional method employs a circuit structure that removes a mismatch in the output end of the receiver and thus, there is difficulty in removing a mismatch of an input signal itself in an input end or a mismatch due to a low noise amplifier (LNA).
In the conventional method, the receiver operates each auxiliary path amplifier formed of an independent bias based on a scheme in which the input signal is cross-coupled in two signal paths. The cross-coupling of the input signal increases a gain, and removes a 3rd harmonic component by adjusting a bias of an auxiliary path and the like.
However, to receive a narrowband signal such as a 2G signal and a wideband signal such as a 3G signal through the same path, a high level of IIP2 (second-order input intercept point) performance is required. The IIP2 is a parameter indicating a linearity of a circuit. In general, a received signal may be distorted by interference between channels having a small frequency difference or mutual interference between signals in a signal band. The IIP2 is defined to indicate a linearity of a circuit based on a relationship between an amount of distortion by a secondary intermodulation (IM2) and an amount of amplified input frequency from among factors causing distortion of a received signal.
Specifically, when power of an input signal is continuously increased in a receiver, power of an IM2 distorted signal increases with a sharp inclination. A power point, where the input signal and the IM2 distorted signal are expected to cross, viewed from an input end of the receiver is defined to be the IIP2. Therefore, the IIP2 is required to be high to secure a high linearity in the wireless communication system, which indicates that the IM2 distortion is minimized.
Therefore, the receiver in the wireless communication system is required to be designed to have a high IIP2 parameter value.
A decrease in the IIP2 parameter value may occur due to the following four mismatch cases.
1. a mismatch of an input signal itself (mostly external component filter (for example, SAW filter), PCB line, Packaging Effect, Bonding inductor, etc).
2. a mismatch of a low noise amplifier (LNA) circuit itself in an input end
3. a mismatch of a mixer circuit itself and a Local Oscillation (LO) signal
4. a mismatch of a Trans-Impedance Amplifier (TIA) itself
The mismatches of the item 1 and the item 2, that is, the mismatch of an input signal itself and the mismatch of a low noise amplifier itself in an input end, may be amplified by a low noise amplifier and the amplified mismatch may be partially decreased through an IIP2 calibration circuit connected to a back end of the low noise amplifier in the receiver. However, the decrease is limited.
Another conventional method of decreasing a mismatch occurring in the input end of the receiver adjusts a direct current at an intermediate point between the input end and the output end by monitoring a current voltage at the output end. However, the method has a limitation in decreasing a mismatch using only a direct current bias.
Also, transmission of a wideband signal (for example, a signal of a 3G or 4G network) for high-speed packet data service and the like has been dramatically increased. However, when interference cancellation, such as removal of a mismatch, is identically performed with respect to the wideband signal and a narrowband signal (for example, a signal of 2G network) for a voice service and the like, effective interference cancellation performance may not be secured.